Since the Reaper Splitter in Siberia's biohazard container is over 10,000 years old, and the Lost Temple of T'hochek harbors a jade crystal artifact that was "ancient" in 1920, I think its probably safe to say humans earlier than the Colonel or spring-heeled Jacque had encountered Splitters. As far as the characters we know in the game though, the Colonel is probably earliest.
I like to think they are alternate reality versions of the same character. You know, like evil Spock with the beard. Two different timelines, one basic character, with some crucial differences, foremost being the precise native timezone. Although, the "hero" characters have noted lineages (for example the Nadirs or the Joneses), so its entirely possible Big Tony is simply Mr. Big's unacknowledged ancestor within the same timeline.
To elaborate a bit, the only talking by the avatar character is during the cutscenes (and these presumably are always once Cortez is there). During the level however, during gameplay, player/Cortez/avatar does not talk.